The invention relates to sensor arrays for detecting analytes, and in particular to an array of sensors with surface modifications to change sensor responses to analytes.
Resonant sensors use target molecules adsorbed in a sensing material to change properties that are reflected in the resonance frequencies of the sensors. A wide variety of cantilever, membrane and piezoelectric resonator-based sensors have been fabricated using MEMS technology. These sensors generally detect agents through the use of polymer films and coatings with selective adsorption for a specific agent or set of agents. Although these sensors provide a certain degree of sensitivity, it is desirable in many applications to have sensors with even higher sensitivities.
A capacitive micromachined ultrasonic transducer (cMUT) is a micromachined device having a substrate and a membrane supported above the substrate by an insulating material. A variable voltage applied between the substrate and membrane drives the membrane to vibrate and emit sound waves at ultrasonic frequencies. Arrays of cMUTs have been used for transmitting and receiving ultrasonic beam patterns in air and water over a frequency range from 10 kHz to 100 MHz. These cMUTs rely on the very large electric field in the gap of the capacitor to provide an electromechanical coupling coefficient close to unity.
cMUTs are mostly used for medical imaging. In addition, they have been used to indirectly measure various fluid characteristics, based on processing of ultrasonic signals transmitted and received through the fluid. In current cMUT devices and applications, the cMUT elements are used to transmit and/or receive ultrasonic energy between the cMUT element and the environment. Moreover, to ensure reliable and consistent operation, cMUT element membranes are normally designed to be non-reactive to chemicals, light, and other environmental factors that may alter or interfere with their operational characteristics. However, due to their resonant character, cMUT devices have the potential to be used as sensors, in a manner similar to MEMS cantilever, membrane, and piezoelectric resonator-based sensors.
One use of cMUT devices in an array of sensors is disclosed in U.S. Pat. No. 7,305,883 to Khuri-Yakub. Sensor elements include a functionalized membrane supported over a substrate by a support frame. The sensor element is connected to an electrical circuit, which is configured to operate the sensor element at or near an open circuit resonance condition. The mechanical resonance frequency of the functionalized membrane is responsive to binding of an agent to the membrane. The exterior surface of each sensor membrane is chemically functionalized to have an affinity for one or more specific, predetermined chemicals. A detector provides a sensor output responsive to the mechanical resonance frequency of the sensor element. A potential disadvantage to this approach is that it may be difficult, time consuming, and/or expensive to chemically functionalize each membrane in the array with different polymer materials to attract different analytes. This is difficult due to the tiny size of the membranes and the limitations of droplet technology for placing drops of different polymers on the membranes to adsorb or bind different target molecules.
US patent application 20130098141 to McCaig discloses cantilever chemical vapor sensors. The sensors are tailored to respond in frequency by controlling the location of deposition of an adsorbing layer, using a gold layer to promote deposition of the adsorbing layer of a polymeric material in a desired location. A thin film of chromium is deposited by thermal evaporation on regions of the device where polymer film growth is to be suppressed or inhibited. Electron beam lithography is used to pattern the chromium masking layer on top of the gold layer before the cantilevers are suspended using a plasma etch. Localization of polymer coating is achieved utilizing a combination of surface initiated atom transfer polymerization (SI-ATRP) and disulfide self-assembled monolayer (SAM) formation on gold. The polymerization initiator contains a disulfide which will adhere to the gold surface, and polymerization only occurs (or very preferentially occurs) at those locations. SI-ATRP is then used to grow a polymer film which is localized to the bare gold surface.